Is there anyway to shield my soundcard? I have a Diamond MX300, and using RMAA, I determined placing it in the bottom PCI slot, interference harmonics dropped the lower it got. So my question is, is there a certain material that will block out the rest, or is it coming the PCI bus, or what? All ideas welcome

Hmmm.. that's an interesting project. Electromagnetic radiation is coming from all electronic devices inside your PC, but mostly from the CPU, the graphics card processor and other chips. Maybe you could build a cardboard enclosure around your soundcard and wrap it with several layers of aluminum foil?

edit: And ground the aluminum foil (with a short cable to the case).

and ensure it isn't liable to ground out anything else

I've had a similar situation with my old screwed-up mobo. Experimenting with shielding didn't help much, so the problem was solved by buying a new board :) If you're seeing spikes at around 50/60 khz and its harmonics, make sure your loopback cable doesn't run near something like a transformer or a poorly shielded power supply. Good luck.

In my experience, domestic aluminium foil doesn't help, even using several layers and grounding them with a thick cable to the comp. case, in fact it increases interference. I guess something more solid and very well grounded (with as much contact to ground as possible) might work better. Maybe to use as ground a 0 voltage reference point at the motherboard instead the of the ground case would work better too.

I've found that most of the times keeping the card as far as possible from other cards and/or having as less cards as possible helps. Having two soundcards inserted usually worsens the problem, since they work at similar frequencies and cause interference in the audio band.